Other options like Plain Colour work fine, so at least I know the Settings Module is doing something.

I think I had this issue in the past and it turned out to be a permission issue. So I've tried changing the permissions of the file located in my home directory to 777, setting owner as root:root. I found where the system wallpapers live (/usr/share/wallpapers/Next/contents/images) and tried dumping my wallpaper in there and setting the permissions the same as the ones that are already there.

I've also tried looking through the logs (syslog / journalctl -f | grep kde) and I don't see any errors

Tried running the screenlocker via terminal and setting the Lock Screen image for some output:

kcmshell5 screenlockerqrc:/kscreenlocker-kcm-resources/wallpaperconfig.qml:50:5: QML StackView: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.qrc:/kscreenlocker-kcm-resources/lnfconfig.qml:50:5: QML StackView: Detected anchors on an item that is managed by a layout. This is undefined behavior; use Layout.alignment instead.kf5.kpackage: No metadata file in the package, expected it at: "/home/user/Pictures/"kf5.kpackage: No metadata file in the package, expected it at: "/home/user/Pictures/"kf5.kpackage: No metadata file in the package, expected it at: "/home/user/Pictures/"org.kde.kcoreaddons: KDirWatch: "/home/user/Pictures" is a directory. Use addDir!org.kde.kcoreaddons: KDirWatch: "/usr/share/wallpapers/Next" is a directory. Use addDir!kf5.kpackage: No metadata file in the package, expected it at: "/usr/share/wallpapers/Next/contents/images/"kf5.kpackage: No metadata file in the package, expected it at: "/usr/share/wallpapers/Next/contents/images/"kf5.kpackage: No metadata file in the package, expected it at: "/usr/share/wallpapers/Next/contents/images/"

Also tried running the screensaver via a terminal for output but got none: